Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru
The coastal upwelling system off the coast of Peru is characterized by high biological activity and a pronounced subsurface oxygen minimum zone, as well as associated emissions of atmospheric trace gases such as N<sub>2</sub>O, CH<sub>4</sub> and CO<sub>2</sub>. F...
| Main Authors: | , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-04-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/13/2453/2016/bg-13-2453-2016.pdf |
| _version_ | 1811239217545609216 |
|---|---|
| author | L. Galgani A. Engel |
| author_facet | L. Galgani A. Engel |
| author_sort | L. Galgani |
| collection | DOAJ |
| description | The coastal upwelling system off the coast of Peru is characterized by high biological
activity and a pronounced subsurface oxygen minimum zone, as well as
associated emissions of atmospheric trace gases such as N<sub>2</sub>O, CH<sub>4</sub>
and CO<sub>2</sub>. From 3 to 23 December 2012, R/V <i>Meteor</i> (M91) cruise
took place in the Peruvian upwelling system between 4.59 and
15.4° S, and 82.0 to 77.5° W. During M91 we
investigated the composition of the sea-surface microlayer (SML), the
oceanic uppermost boundary directly subject to high solar radiation, often
enriched in specific organic compounds of biological origin like
chromophoric dissolved organic matter (CDOM) and marine gels. In the SML,
the continuous photochemical and microbial recycling of organic matter may
strongly influence gas exchange between marine systems and the atmosphere.
We analyzed SML and underlying water (ULW) samples at 38 stations focusing
on CDOM spectral characteristics as indicator of photochemical and microbial
alteration processes. CDOM composition was characterized by spectral slope
(<i>S</i>) values and excitation–emission matrix fluorescence (EEMs), which allow
us to track changes in molecular weight (MW) of DOM, and to determine potential
DOM sources and sinks. Spectral slope <i>S</i> varied between 0.012 to 0.043 nm<sup>−1</sup>
and was quite similar between SML and ULW, with no significant
differences between the two compartments. Higher <i>S</i> values were observed in
the ULW of the southern stations below 15° S. By EEMs, we
identified five fluorescent components (F1–5) of the CDOM pool, of which two
had excitation/emission characteristics of amino-acid-like fluorophores (F1,
F4) and were highly enriched in the SML, with a median ratio SML : ULW of 1.5
for both fluorophores. In the study region, values for CDOM absorption
ranged from 0.07 to 1.47 m<sup>−1</sup>. CDOM was generally highly concentrated in
the SML, with a median enrichment with respect to the ULW of 1.2. CDOM
composition and changes in spectral slope properties suggested a local
microbial release of DOM directly in the SML as a response to light exposure
in this extreme environment. In a conceptual model of the sources and
modifications of optically active DOM in the SML and underlying seawater
(ULW), we describe processes we think may take place (Fig. 1); the production of CDOM of higher MW by microbial release through
growth, exudation and lysis in the euphotic zone, includes the identified
fluorophores (F1, F2, F3, F4, F5). Specific amino-acid-like fluorophores
(F1, F4) accumulate in the SML with respect to the ULW, as photochemistry
may enhance microbial CDOM release by (a) photoprotection mechanisms and (b) cell-lysis processes. Microbial and photochemical degradation are potential
sinks of the amino-acid-like fluorophores (F1, F4), and potential sources of
reworked and more refractory humic-like components (F2, F3, F5). In the
highly productive upwelling region along the Peruvian coast, the interplay
of microbial and photochemical processes controls the enrichment of
amino-acid-like CDOM in the SML. We discuss potential implications for
air–sea gas exchange in this area. |
| first_indexed | 2024-04-12T12:55:53Z |
| format | Article |
| id | doaj.art-59423911d33646a1a645192226d62dfa |
| institution | Directory Open Access Journal |
| issn | 1726-4170 1726-4189 |
| language | English |
| last_indexed | 2024-04-12T12:55:53Z |
| publishDate | 2016-04-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Biogeosciences |
| spelling | doaj.art-59423911d33646a1a645192226d62dfa2022-12-22T03:32:20ZengCopernicus PublicationsBiogeosciences1726-41701726-41892016-04-011382453247310.5194/bg-13-2453-2016Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of PeruL. Galgani0A. Engel1GEOMAR – Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, GermanyGEOMAR – Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, GermanyThe coastal upwelling system off the coast of Peru is characterized by high biological activity and a pronounced subsurface oxygen minimum zone, as well as associated emissions of atmospheric trace gases such as N<sub>2</sub>O, CH<sub>4</sub> and CO<sub>2</sub>. From 3 to 23 December 2012, R/V <i>Meteor</i> (M91) cruise took place in the Peruvian upwelling system between 4.59 and 15.4° S, and 82.0 to 77.5° W. During M91 we investigated the composition of the sea-surface microlayer (SML), the oceanic uppermost boundary directly subject to high solar radiation, often enriched in specific organic compounds of biological origin like chromophoric dissolved organic matter (CDOM) and marine gels. In the SML, the continuous photochemical and microbial recycling of organic matter may strongly influence gas exchange between marine systems and the atmosphere. We analyzed SML and underlying water (ULW) samples at 38 stations focusing on CDOM spectral characteristics as indicator of photochemical and microbial alteration processes. CDOM composition was characterized by spectral slope (<i>S</i>) values and excitation–emission matrix fluorescence (EEMs), which allow us to track changes in molecular weight (MW) of DOM, and to determine potential DOM sources and sinks. Spectral slope <i>S</i> varied between 0.012 to 0.043 nm<sup>−1</sup> and was quite similar between SML and ULW, with no significant differences between the two compartments. Higher <i>S</i> values were observed in the ULW of the southern stations below 15° S. By EEMs, we identified five fluorescent components (F1–5) of the CDOM pool, of which two had excitation/emission characteristics of amino-acid-like fluorophores (F1, F4) and were highly enriched in the SML, with a median ratio SML : ULW of 1.5 for both fluorophores. In the study region, values for CDOM absorption ranged from 0.07 to 1.47 m<sup>−1</sup>. CDOM was generally highly concentrated in the SML, with a median enrichment with respect to the ULW of 1.2. CDOM composition and changes in spectral slope properties suggested a local microbial release of DOM directly in the SML as a response to light exposure in this extreme environment. In a conceptual model of the sources and modifications of optically active DOM in the SML and underlying seawater (ULW), we describe processes we think may take place (Fig. 1); the production of CDOM of higher MW by microbial release through growth, exudation and lysis in the euphotic zone, includes the identified fluorophores (F1, F2, F3, F4, F5). Specific amino-acid-like fluorophores (F1, F4) accumulate in the SML with respect to the ULW, as photochemistry may enhance microbial CDOM release by (a) photoprotection mechanisms and (b) cell-lysis processes. Microbial and photochemical degradation are potential sinks of the amino-acid-like fluorophores (F1, F4), and potential sources of reworked and more refractory humic-like components (F2, F3, F5). In the highly productive upwelling region along the Peruvian coast, the interplay of microbial and photochemical processes controls the enrichment of amino-acid-like CDOM in the SML. We discuss potential implications for air–sea gas exchange in this area.http://www.biogeosciences.net/13/2453/2016/bg-13-2453-2016.pdf |
| spellingShingle | L. Galgani A. Engel Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru Biogeosciences |
| title | Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru |
| title_full | Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru |
| title_fullStr | Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru |
| title_full_unstemmed | Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru |
| title_short | Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru |
| title_sort | changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated dom turnover in the upwelling region off the coast of peru |
| url | http://www.biogeosciences.net/13/2453/2016/bg-13-2453-2016.pdf |
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